Changes in directory llvm/lib/Target/PowerPC:
PPCISelLowering.cpp updated: 1.225 -> 1.226
---
Log message:
lower "X = seteq Y, Z" to '(shr (ctlz (xor Y, Z)), 5)' instead of
'(shr (ctlz (sub Y, Z)), 5)'.
The use of xor better exposes the operation to bit-twiddling logic in the
dag combiner. For example, this:
typedef struct {
unsigned prefix : 4;
unsigned code : 4;
unsigned unsigned_p : 4;
} tree_common;
int foo(tree_common *a, tree_common *b) {
return a->code == b->code;
}
Now compiles to:
_foo:
lwz r2, 0(r4)
lwz r3, 0(r3)
xor r2, r3, r2
rlwinm r2, r2, 28, 28, 31
cntlzw r2, r2
srwi r3, r2, 5
blr
instead of:
_foo:
lbz r2, 3(r4)
lbz r3, 3(r3)
srwi r2, r2, 4
srwi r3, r3, 4
subf r2, r2, r3
cntlzw r2, r2
srwi r3, r2, 5
blr
saving a cycle.
---
Diffs of the changes: (+5 -3)
PPCISelLowering.cpp | 8 +++++---
1 files changed, 5 insertions(+), 3 deletions(-)
Index: llvm/lib/Target/PowerPC/PPCISelLowering.cpp
diff -u llvm/lib/Target/PowerPC/PPCISelLowering.cpp:1.225
llvm/lib/Target/PowerPC/PPCISelLowering.cpp:1.226
--- llvm/lib/Target/PowerPC/PPCISelLowering.cpp:1.225 Mon Nov 13 19:38:31 2006
+++ llvm/lib/Target/PowerPC/PPCISelLowering.cpp Mon Nov 13 23:28:08 2006
@@ -1038,12 +1038,14 @@
}
// If we have an integer seteq/setne, turn it into a compare against zero
- // by subtracting the rhs from the lhs, which is faster than setting a
- // condition register, reading it back out, and masking the correct bit.
+ // by xor'ing the rhs with the lhs, which is faster than setting a
+ // condition register, reading it back out, and masking the correct bit. The
+ // normal approach here uses sub to do this instead of xor. Using xor
exposes
+ // the result to other bit-twiddling opportunities.
MVT::ValueType LHSVT = Op.getOperand(0).getValueType();
if (MVT::isInteger(LHSVT) && (CC == ISD::SETEQ || CC == ISD::SETNE)) {
MVT::ValueType VT = Op.getValueType();
- SDOperand Sub = DAG.getNode(ISD::SUB, LHSVT, Op.getOperand(0),
+ SDOperand Sub = DAG.getNode(ISD::XOR, LHSVT, Op.getOperand(0),
Op.getOperand(1));
return DAG.getSetCC(VT, Sub, DAG.getConstant(0, LHSVT), CC);
}
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